Mono-grain layer membrane

ABSTRACT

A membrane comprising a film of synthetic resin in which a piece of wire gauze is embedded. The apertures of the piece of wire gauze accommodate grains which protrude through the surface on one or both sides of the film. Such a membrane may be used as a partition between two compartments. Dependent on the properties of the grains, molecules, ions and/or electrons may be transported selectively from one compartment to the other.

ited States Patent 1 1 Oomen et al.

1111] 3,787,277 Jan. 22, 1974 MONO-GRAIN LAYER MEMBRANE Inventors:.loris Jan Cornelis Oomen; Donald Robert Wolters, both of Emmasingel,Eindhoven, Netherlands us. Philips Corporation, New York, N.Y.

Filed: May 12, 1972 Appl. No.: 252,669

Related Application Data Continuation of Ser. No. 71,467, Sept. 11,1970, abandoned.

Assignee:

Foreign Application Priority Data Sept. 18, 1969 Netherlands 6914202 us.Cl 161/87, 5 5/524, 55/528,

117/25, 96/44 1m. 01 u s 16 Field of Search 161/92, 93, 94, 95,87,88,

w v v References Cited UNITED STATES PATENTS 2,386,780 10/1945 Cross161/87 543,964 8/1895 Michell 161/87 637,509 11/1899 Henay 161/873,322,608 5/1967 Mason et a1. 161/87 3,171,772 3/1965 Lomar et a1.161/87 3,474,600 10/1969 Tobias 55/524 3,019,127 1/1962 Czerwonka et a155/524 Primary Examiner-George F. Lesmes Assistant Examiner-William R.Dixon, Jr. Attorney, Agent, or Firm-Frank R. Trifari [57] ABSTRACT Amembrane comprising a film of synthetic resin in which a piece of wiregauze is embedded. The-apertures of the piece of wire gauze accommodategrains which protrude through the surface on one or both sides of thefilm. Such a membrane may be used as a partition between twocompartments. Dependent'on the properties of the grains, molecules, ionsand/or electrons may be transported selectively from one compartment tothe other.

3 Claims, 5 Drawing Figures MONO-GRAIN LAYER MEMBRANE This is acontinuation of application Ser. No. 071 ,467 filed Sept. ll, 1970, nowabandoned.

The invention relates to a monograin layer membrane comprising a singlelayer of grains which are embedded in a synthetic resin foil in whichpart of the surface of the grains on at least one side of the foil isnot coated with a synthetic resin.

Such a membrane may be obtained as follows.

The grains to be embedded are scattered on a surface provided with atacky layer. Those grains which are not fixed to the surface by thetacky layer are removed. A solution of a synthetic resin is provided ina thin layer on the single layer of grains left on the surface. Thesolvent is subsequently removed, for example, by heating the assembly.In this way a synthetic resin foil is formed. It is alternativelypossible to use a solution of materials which form a synthetic resin asa result of chemical reaction after removal of the solvent. Heat may berequired for removal of the solvent or for the chemical reaction.

The synthetic resin film with the embedded grains is subsequentlydetached from the surface, for example, by dissolving the adhesivelayer. The result then is a membrane in which part of the surface of thegrains on one side, namely that part of the surface by which the grainshave adhered to the adhesive layer, is not coated with synthetic resin.

If membranes are desired in which on both sidespart of the surface ofthe grains is free from synthetic resin the synthetic resin foil isremoved to the necessary depth by partly etching or dissolving the resinfoil away on that side of the membrane where the surface of the grainsis to be freed from synthetic resin.

A membrane in which on-both sides part of the surface of the grains isnot coated with synthetic resin may alternatively be obtained by Achoosing the circumstances in the manufacture of the synthetic resinfoil in such a manner that the synthetic resin retracts between thegrains when the solvent is removed.-

Dependent on the properties of the grains which are present in themono-grain layer membranes obtained in accordance with these methods anddependent on the extent of accessibility of the grains, a more or lessselective transport of molecules, ions and/or electrons through themembrane in a direction at right angles to the plane of the membrane ispossible. Membranes of this kind may be used as partitions between twocompartments containing media between which an exchange or transport ofions, molecules and/or electrons must take place. Examples of these usesare partitions in devices for dialysis and'electrodialysis, fuelelements, accumulators, batteries, devices for separating gases, forselective permeation as in filtering devices and devices for reverseosmosis. Electron-conducting membranes may be used as electrodes inbatteries. semiconducting membranes may be used in solar cells and othersemiconductor devices.

Particularly in those cases where a difference in pressure existsbetween the compartments which are separated by such membranes it isdesirable to have strong and also flexible membranes available at thelowest possible resistance to transport of the molecules, ions and/orelectrons.

The amount of molecules, ions and/or electrons which can be transportedby the membrane per cm depends on the amount of grains, present per cmThe cracking strengthof membranes containing a great amount of grainsmanufactured in accordance with the methods described above is not highenough for certain uses.

It is an object of the present invention to provide a membrane which hasa high mechanical strength and a low transport resistance through themembrane for ions molecules and/or electrons, while the density of thegrains is as high as possible.

According to the invention this object is satisfied by a membrane whichis characterized in that the grains are provided in the apertures in apiece of wire gauze which is embedded in the synthetic film togetherwith the grains.

Such 'a membrane has a high strength because the piece for wire gauzeserves as a reinforcement of the film. In addition the membraneconstruction according to the invention has the great advantage thatdifferences in grain density cannot substantially occur due to thearranging function of wire gauze. Such differences in grain density arepossible in the known constructions. The construction renders it alsopossible to provide the grains or different kinds of grains in themembrane according to certain patterns.

It is readily evident that grains of any kind may be present in themembrane according to the invention and that the construction as such isindependent of the nature of the grains. The grains may consist of, forexample: ion-exchange material such as cation-exchange or anion-exchangeresins; electrically conducting materials such as carbon, metals orsemiconducting materials; materials which pass molecules of a certainsize commercially known as molecular sieves or materials whichexclusively pass cations or anions of a certain size. The said materialsare known in the art and are commercially available, a detaileddescription of which may be found, if desired, in the relevantlitterature on this subject.

The piece of wire gauze present in the membrane may be manufactured fromwires of natural or synthetic polymers such as natural silk, nylon,polyester, polyvinylidenechloride, glass fibres, metals or alloys suchas stainless steel, phosphor bronze, nickel and nickel alloys.Combinations of metal wires and wires of other materials arealternatively possible. The pieces of wire gauze may be obtained byweaving or by interconnecting, for example, by means of knotting,welding or otherwise attaching two or more layers of crossed parallelwires at their points of intersection, the wires being locatedrelatively to one another at a given angle in the successive layers.Pieces of wire gauze from synthetic resin may alternatively be obtainedby extrusion. The apertures in the wire gauze may be, for example,square or hexagonal. The piece of wire gauze may have apertures ofmutually different shapes and dimensions which may each comprise, forexample, a grain material having a different function. In thisconnection a piece of wire gauze is also understood to mean a materialobtained by the provision of a large number of apertures in a metal orsynthetic resin film.

The membrane preferably comprises a piece of wire gauze whose apertureshave dimensions such that they provide room for one or more grains. Amaximum strength of the membrane is obtained in a preferred embodimentin which each aperture accommodates one grain whose diameterapproximately corresponds to the dimensions of the aperture.

The membranes according to the invention may be manufactured inaccordance with several methods which are different in detail but mainlyconsist of two manufacturing stages.

The first manufacturing stage generally consists in a temporary fixationof the grains in the apertures of the piece of wire gauze. During thesecond manufacturing stage the grains together with the piece of wiregauze are combined to form an assembly while forming a film with the aidof a synthetic resin. If during the latter stage a great part of thesurface of the grains which is sufficient for transport through themembrane does not remain free from synthetic resin, part of thesynthetic resin may be removed again in a possible third manufacturingstage.

The temporary fixation of the grains in the apertures of a piece of wiregauze may be effected, for example, by providing an adhesive layer on aflat surface, spreading a piece of wire gauze on the adhesive layer andsubsequently scattering grains on the piece of wire gauze. Part of thegrains fills the apertures in the piece of wire gauze and adheres to theadhesive layer, the other grains may easily be removed by brushing themoff or sucking them up.

In this modification of the method the surface of a glass or metal platemay be used, for example, as the flat surface. A thin layer of adhesiveis spread on the surface. It has been found that suitable adhesives forthis purpose consist of highly viscous solutions of natural andsynthetic rubbers, for example polyisoprene.

It is possible to use commercially available adhesive films for thispurpose.

The adhesive layer may alternatively be obtained with the aid of apositive or negative photoresist. To this end a layer of photoresist isprovided on the flat surface. The photoresist is subsequently exposed inaccordance with a certain pattern and is then developed. The parts ofthe photoresist left on the surface are somewhat tacky after developmentand before drying, or may be made tacky with the aid of a solvent. Thecommercially available photoresists may be used for this purpose.

After the grains are fixed in the apertures of the piece of wire gauzeand the grains which have not adhered to the tacky surface are removed,a solution of a synthetic resin or of materials which form a syntheticresin after a chemical reaction, is poured out in a thin layer onto thepiece of wire gauze. The solvent is subsequently removed, for example,by heating the assembly to form a film consisting of the relevantsynthetic resin.

All synthetic resins forming a film under these circumstances are inprinciple suitable for this purpose. Examples of such synthetic resinsare inter alia: polyester resins, polycarbonate resins, polyamideresins, phenolformaldehyde resins, polyurethane resins and epoxy resins.

Unless this method is performed in such a manner that the syntheticresin retracts between the grains upon formation, it may be necessarythat part of the film is removed after the formation of the film so thatpart of the surface of the grains is free from synthetic resin. This maybe effected by scouring the film, dissolving it partially, saponifyingit or etching it away with the aid of an acid with or without thepresence of an oxidant. According to a further modification of themethod of manufacturing a membrane according to the invention a piece ofwire gauze is used whose wires are already coated with a thermoplasticsynthetic resin or a thermo-setting synthetic resin which is in aso-called- B-stage so that it first melts or softens upon heating andthen forms a coherent foil before hardening. The previously mentionedsynthetic resins for the manufacture of the synthetic resin film mayalso be used for this purpose.

The piece of wire gauze is laid on an adhesive layer in the mannerpreviously described, grains are scattered on the piece of wire gauzeand the superfluous grains are removed. The membrane is then obtained byheating the assembly during which the synthetic resin on the piece ofwire gauze softens and forms a coherent film which also hardens if athermosetting synthetic resin is used..

The synthetic resin on the piece of wire gauze may be used to fix thegrains temporarily when these are provided. To this end the syntheticresin is made tacky, for example, by treating it with a solvent or withthe vapors of a solvent. After providing the grains the solvent isremoved, for example, by heating and the synthetic resin film is formed.It is of course alternatively possible to coat the piece of wire gauzewith a tacky substance (adhesive), to accommodate the grains in theapertures and subsequently to form the synthetic resin film by coatingthe assembly with a solution of a synthetic resin as previouslydescribed.

The grains may be provided in the membrane according to certain patternsby using a photoresist. This may be effected in several manners.

For example, the apertures in certain parts of the piece of wire gauzemay be closed according to a certain pattern. To this end the piece ofwire gauze is coated, for example, with a photoresist. The resist layeris exposed in accordance with the desired pattern and is subsequentlydeveloped. Grains may then be provided in the apertures which are notclosed by the photoresist by means of any of the methods previouslydescribed.

It is alternatively possible to fix the grains in the piece of wiregauze according to a certain pattern by means of a photomechanicalmethod.

In that case operations may, for example, be carried out as follows. Thepiece of wire guaze is immersed in a photoresist and is retreatedtherefrom so slowly that a thin, homogeneous resist is left on the pieceof wire gauze. After drying a structure is usually obtained in whichonly the wires are coated with a photoresist. The piece of wire gauze isthen exposed in accordance with the desired pattern, whereafter thephotoresist is developed with a solvent. Photoresist is then left on thedesired wires. Upon development the insoluble part of the photoresistusually becomes somewhat tacky, but if this might not be the case asolvent may generally be found by which the resist can be made tacky.

The grains are then provided on the areas in the piece of wire gauzewhich have been made tacky;

It is of course alternatively possible to provide the photoresist on asubstrate, to subsequently expose the photoresist in accordance with acertain pattern and thereafter to develop it. The piece of wire gauze isspread on the substrate on which the tacky photoresist has been providedaccording to a certain pattern. Grains are sprinkled on the piece ofwire gauze. The grains which have not adhered to the substrate areremoved. Subsequently, the monograin layer membrane is finished inaccordance with any of the previously described methods. A method ofmanufacturing a membrane according to the invention will now bedescribed more fully with reference to the accompanying drawmg;

In the drawing FIGS. 1 to 5 illustrate cross-sectional views ofdifferent stages of this method.

A tacky layer 2 is provided on a substrate 1. A piece of wire gauze 3 isspread'on this layer. The wire gauze consists .of two layers of mutuallyparallel wires which intersect each other at an angle of 90. The wiresare welded together at their points of intersection.

Grains 4 are scattered on the piece of wire gauze, the largest dimensionof the grains being somewhat smaller than the smallest diameter of theapertures in the piece of wire gauze (in this case slightly smaller thanthe side of the square apertures in the piece of wire gauze). A numberof these grains 4 fall into the apertures in the piece of wire gauze 3and are held by the tacky layer '2. After removal of superfluous grainsthe structure is obtained which is shown in a perspective side elevationin FIG. 2. A solution of a synthetic resin 5 is poured out onto thegrains 4 (FIG. 3). When removing the solvent the layer 5 shrinks andretracts between grains 4. When correctly choosing the circumstances(which must be experimentally determined from case to case) it maybeachieved that the ultimately formed synthetic film 6 does not cover thegrains over part of their surface on the side remote from thesubstrate 1. The membrane (consisting of the grain 4 and the piece ofwire gauze 3 embedded in the synthetic film 6) is then detached from thesubstrate.

This may be effected, for example, by dissolving the tacky layer 2 in asolvent which does not affect the synthetic film. A membrane as is shownin a cross-. sectional side elevation in FIG. 5 is ultimately obtained.

Several embodiments of membranes according to the invention will nowfurther be described with reference to the following examples, wheredimensions are expressed in microns (pm for micrometers).

- EXAMPLE I.

A piece of wire gauze having square apertures of 60 um X 60 ummanufactured from nylon wires, diameter of wires: um, was covered on oneside with an adhesive film consisting of a flexible supportprovided-with a layer of polyisoprene rubber. Grains having a diameterof between 37 and 44 am of a cation-exchange material mainly consistingof a styrenedivinylbenzene copolymer were scattered on the side of thepiece of wire gauze remote from the adhesive film. The grains which didnot adhere to the adhesive film were subsequently brushed off. Theassembly forming the piece of wire gauze, adhesive film and grains leftthereon was subsequently immersed in a solution comprising:

500 g Desmopheen I200 (Bayer) 600 g Desmodur L (Bayer) 400 gethylacetate Desmopheen 1200 is a branched polyester obtained bycondensation of adipic acid propanetriol and butylene glycol. Desmodur Lcontains the addition product of 2,2- dioxymethyl butanol l incuding amixture of 2,4 toluene diisocyanate and 2,6'- toluene diisocyanate (65:

The assembly was subsequently lifted out of the solution and heated for16 hours at l00C The solvent was then removed and a film of polyurethaneresin was formed from the above-mentioned components. Part of thesurface of the grains were not coated with polyurethane resin. Theadhesive film wasthen removed by immersing the assembly in xylene duringwhich the polyisoprene rubber partially dissolved and lost its adhesivestrength. The membrane obtained in this manner had a permselectivitywhich is better than 99 percent. The electric resistance wasapproximately 9 Q sq.cm.

EXAMPLE II.

A piece of wire gauze consisting of nylon wires (diameter 30 ,urn)having square apertures of 60 um was immersed in a polyisoprene solution(10 gms of polyisoprene per 100 mls of xylene) and was subsequentlyretreated from this solution so slowly that a thin, homogeneous layercovered the piece of wire gauze. The solvent was evaporated at 20Cambient temperature. A piece of wire gauze was obtained whose wires areenveloped by a thin film of polyisoprene. Grains comprising sulphonatedstyrenedivinylbenzene copolymer having cation-exchange properties weresubsequently scattered on the piece of wire gauze. The grains had a sizeof between 36 um and 45 pm. The grains were pressed into the aperturesof the piece of wire gauze with the aid of a roller. A coherent film wasformed after 16 hours of heating at 100C. Part of the surface of thegrains protruded on either side from the film surface.

EXAMPLE III A thin layer of a commercially available photoresist wasspread on the surface of a glass plate. The photoresist was exposed inaccordance with a desired pattern while using a mask. The exposed layerwas subsequently developed and post-treated as prescribed by themanufacturer of the resist. Subsequently the photoresist patterns lefton the glass plate were made tacky by means of a solvent, in this caseby xylene. A piece of wire gauze was then spread on the glass plate. Themembrane was further obtained in the manner as described in Example I.The ultimate result was a membrane comprising the grains distributed inaccordance with a certain pattern.

What is claimed is:

l. A mono-grain layer membrane which comprises a piece of wire gauze; alayer of grains having a thickness of one grain diameter disposed in theapertures of said wire gauze, the dimensions of each of said grainsbeing such that said grains fit within the area created by saidapertures, the smallest dimension of each of said grains being largerthan the diameter of the wire of said wire gauze; a syntheticresin'di'sposed between said grains and provided within said apertures,said synthetic resin binding said grains and said wire gauze togetherand wherein part of the surface of said grains on at least one side ofsaid membrane are not coated with said synthetic resin.

2. A mono-grain layer membrane as claimed in claim 1, wherein said wiregauze has apertures which provide room for only one of said grains.

3. A mono-grain layermembrane comprising a wire gauze, the apertures ofwhich have disposed therein grains, said grains and wire gauze beingembedded in a synthetic resin film.

2. A mono-grain layer membrane as claimed in claim 1, wherein said wiregauze has apertures which provide room for only one of said grains.
 3. Amono-grain layer membrane comprising a wire gauze, the apertures ofwhich have disposed therein grains, said grains and wire gauze beingembedded in a synthetic resin film.